The present invention relates to a pressure-sensitive adhesive for a pressure-sensitive adhesive layer in a volume hologram laminate which can reduce spotty hologram defects caused in a stacked and/or pressed state in transit or during storage in warehouses or the like.
Conventional volume hologram laminates generally have a construction comprising a substrate and, stacked on the substrate in the following order, a pressure-sensitive adhesive layer, a volume hologram layer, a pressure-sensitive adhesive layer, and a transparent protective film. Volume hologram laminates applied to various forms of substrates, such as identification cards or papers, admission tickets for examination, ID cards, booklets such as passports, have been developed. However, volume hologram laminates using, as a dry volume phase type hologram recording material, a recording material comprising a matrix polymer and a photopolymerizable compound pose a problem of the occurrence of spotty hologram defects when the volume hologram materials applied, for example, to books or the like are kept in a stacked and/or pressed state in transit or during storage in warehouses or the like.
The spotty hologram defects are found when the hologram-recorded layer is observed. The spots are in the form of ellipse having a size of 0.2 to 2 mm in major axis and about 0.1 to 1.5 mm in minor axis, or in the form of a circle having a radius of about 0.1 to 2 mm, and occur in a regularly arranged state. The hologram defects depend upon a light source or a viewing angle. Therefore, the hologram defects are considered as holographic defects rather than as defects attributable to the inclusion of air bubbles or foreign matter in the volume hologram layer.
Accordingly, it is an object of the present invention to provide a pressure-sensitive adhesive for a pressure-sensitive adhesive layer in a volume hologram laminate which, when kept in the pressed state, for example, during storage, is less likely to cause spotty hologram defects in the volume hologram layer.
According to one aspect of the present invention, there is provided a pressure-sensitive adhesive for a pressure-sensitive adhesive layer in a volume hologram laminate comprising a substrate and, stacked on the substrate in the following order, a first pressure-sensitive adhesive layer, a volume hologram layer, a second pressure-sensitive adhesive layer, and a surface protective film, said second pressure-sensitive adhesive layer having been formed from an acrylic pressure-sensitive adhesive comprising an acrylic copolymer resin, composed mainly of an alkyl acrylate, and a crosslinking agent, said second pressure-sensitive adhesive layer having a dynamic storage modulus of not less than 2.5xc3x97105 Pa as measured at a measuring frequency of 6.28 rad/sec and a temperature of 50xc2x0 C. and a loss tangent (tan xcex4) defined as dynamic loss modulus/dynamic storage modulus at 120xc2x0 C. of not more than 0.15.
According to a preferred embodiment of the present invention, the crosslinking agent is a crosslinking agent for room temperature crosslinking.
According to another preferred embodiment of the present invention, the crosslinking agent is a crosslinking agent for heat crosslinking.